Ace partner, pure at heart, in buff (7)
I believe the answer is:
amateur
'buff' is the definition.
(thesaurus)
'ace partner pure at heart' is the wordplay.
'ace' becomes 'a'.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'at heart' indicates the central letters.
The middle of 'pure' is 'ur'.
'a'+'mate'+'ur'='AMATEUR'
'in' acts as a link.
(Other definitions for amateur that I've seen before include "Engaged in as a pastime" , "Unpaid or unskilled" , "Unpaid (sport)" , "refusing payment" , "Person who does something as a pastime rather than a profession" .)